Automatic guided vehicles operating without the assistance of a human operator are in common commercial use today. Such vehicles require sophisticated collision detecting devices to prevent contact between the vehicle and objects in the area surrounding the vehicle. Obstacle detection systems of various designs have been employed in the past. Systems utilizing the transmission and reception of radiant energy have proven particularly suitable for this task. For example, U.S. Pat. No. 3,664,701 issued May 23, 1972 to Lewis Kondur is typical of one such system. The Kondur device includes a light source mounted on the vehicle directed toward a retroreflective target mounted on a movable bumper associated with the vehicle. Under normal circumstances, energy from the light source is directed back along the identical path from the target to a light receiving device. In response to contact between the movable bumper and an external object, the retroreflector is moved relative to the light source and the energy is no longer detected by the receiving device.
Bumper systems such as that described above are adequate for detecting certain classes of obstacles. However, such systems fail to detect objects encroaching from the sides or rear of a vehicle. In addition, owing to the high degree of reliability required for obstacle detection systems, it is desirable to employ some form of failure protection apparatus in association with the obstacle detection system.
Some initial attempts to provide such failure protection devices are known in the art. For example, U.S. Pat. No. 3,560,922 issued Feb. 2, 1971 to U.S. Pat. No. 3,560,922 issured Feb. 2, 1971 to Kenneth A. Wilson incorporates a simple contact switch type bumper system. In response to contact between a bumper and an external object, a switch contact is closed, producing a stop signal. The stop signal is also produced in response to failure of one of the wire connections to the bumper switch. Another example of a failure detection system is found in U.S. Pat. No. 4,087,782 issued May 2, 1978 to Kazuo Oishi, et al. A collision detecting system has a bumper inductively coupled to an oscillator circuit. A "checking" switch is employed to selectively operate the detection system in a "check" mode. While in the "check" mode, a warning is provided in response to detecting an open circuit in the lead wires of a sensor and to detecting failure of the oscillator circuit.
None of the prior systems known in the art adequately detects failures associated with an optical collision detection device. Such a failure protection device should advantageously detect both open and short circuit conditions associated with the collision detection system, and should responsively produce a signal indicating such failure. It is further desirable that the failure detection system operate automatically upon each start up of the vehicle.
The present invention is directed to overcoming one or more of the problems as set forth above.